1. Field of the Invention
The present invention relates to an image forming apparatus, and in particular, to an image forming apparatus having a recording head in which plural unit recording heads are lined-up in a direction orthogonal to a conveying direction of a recording medium.
2. Description of the Related Art
Generally, a recording head which is known in an inkjet system has plural nozzles which discharge ink. Ink is discharged from the nozzles by utilizing the vibration of a piezo element or the heat generated by a heat-generating element or the like. Plural recording heads, which discharge inks of the respective colors of yellow, magenta, cyan, black, and the like, are provided. A color image is formed by recording, in a superposed manner, the respective colors of the recording heads.
However, currently, inkjet recording devices which are mainly becoming popular are those of a recording method called serial scanning which records line-by-line by moving a recording head reciprocally while conveying a recording sheet. This method is compact and inexpensive, but has the disadvantage that the recording head must be scanned plural times in order to record an image over the entire recording medium and the recording speed is slow. In order to improve the recording speed, the number of scans must be reduced, and the recording head must be made to be long. The technique which has pushed this to the limit is a non-scanning recording method which records over a length which is substantially the same as the recording width. This recording method is an inkjet recording device having a recording head which corresponds to the width of the recording medium and at which a large number of nozzles are lined-up along a length which is substantially the same as the width of the recording medium. Recording is carried out by moving the recording medium with respect to the recording head which is fixed.
In addition thereto, there has also been proposed, as such a non-scanning recording method, a structure in which plural recording heads such as those used in serial scanning are lined-up in a staggered manner or the like along the width of the recording medium as unit recording heads. The scanning speed can also be improved with this inkjet recording device.
In this way, inkjet recording devices which, while continuously conveying a recording medium, record by non-scanning type recording heads which correspond to the width of the recording medium, have been proposed in order to improve the recording speed and handle application to office use.
On the other hand, although a color image is formed by recording, in a superposed manner, the respective colors of plural recording heads as described above, if offset of the recording positions of the recording heads arises, color offset arises and the image quality deteriorates. Thus, techniques such as those disclosed in Japanese Patent Application Laid-Open (JP-A) No. 04-193542 have been proposed.
In the technique disclosed in JP-A No. 04-193542, in an inkjet recording device having a full-line-type recording head having plural discharge openings over the entire width of the recording region of a recording medium, subscan offset caused by inclining of the nozzles of the respective colors, and color offset arising due to warping of the nozzles, are detected by reading a pattern on a sheet, and the color offset is corrected by changing the writing timing.
As in the invention disclosed in JP-A No. 04-193542, in an image forming device having a full-line-type recording head equipped with plural discharge openings over the entire width of the recording region of the recording medium, the color offset can be corrected merely by changing the writing timing as described above.
However, in a recording head equipped with plural unit recording heads, there is the problem that, due to offset in the assembled positions of the respective unit recording heads or the like, the offset of the recording positions cannot be corrected merely by changing the writing timing.